1. Field of the Invention
The present invention relates to a ballast for a discharge lamp using an inverter circuit.
2. Description of the Prior Art
A circuit as shown in FIG. 11 is conventionally known as a ballast for a discharge lamp, where a chopper circuit 4 and an inverter circuit 5 share a switching device 51. This circuit is disclosed in Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 3-276598 and is designed to have a high power factor as to an input voltage applied to a discharge lamp 6 and reduce harmonic components.
The circuit configuration of the ballast as mentioned above will be described specifically below. Referring to FIG. 11, transistors 50 and 51, which function as first and second switching devices respectively, are connected in series so as to constitute the inverter 5. The inverter 5 is connected in parallel to a smoothing capacitor 55. A driving circuit 8 is connected to the inverter circuit 5 to drive it. A resonance circuit 7 includes a capacitor 52 and an inductor 53 that are connected to the outputs of the inverter circuit 5. A discharge lamp 6 is connected to the output of the resonance circuit 7 and a capacitor 54 is connected in parallel to the discharge lamp 6. The output terminals of a rectifier 3, which is connected to an AC power source 1 via a noise filter 2, are connected to the source and the drain of the transistor 51 via the inductor 41.
Next, the operation of the above-described circuit will be described. An input from the AC power source 1 passes through the filter 2 and is subjected to full-wave rectification in the rectifier 3. The resultant ripple voltage is applied to the chopper circuit 4, so that the smoothing capacitor 55 is charged via the inductor 41. When a DC voltage is applied to the smoothing capacitor 55, the driving circuit 8 supplies a control signal so that the transistors 50 and 51 of the inverter circuit 5 alternate on and off repeatedly.
When the transistors 50 and 51 alternate on and off repeatedly in response to the control signal supplied from the driving circuit 8, an AC voltage is applied to the resonance circuit 7 including the capacitor 52 and the inductor 53 connected to the output terminals of the inverter circuit 5. As a result, a voltage and a current limited by the capacitor 52 or the inductor 53 are supplied to the discharge lamp 6 connected to the output terminal of the resonance circuit 7 so that the discharge lamp 6 lights. The capacitor 54 is connected in parallel to the discharge lamp 6 for the purpose of generating a voltage to start the discharge lamp 6 and stabilizing the lighting circuit.
In the above-described operation, when an instantaneous voltage of the AC power source 1 is low, the increase rate of the current that flows in the inductor 41 is low. Therefore, the energy accumulated in the inductor 41 for a fixed period is small when the transistor 51 turns off, so that the charging voltage to the smoothing capacitor 55 drops. At this time, the input current becomes small in accordance with the instantaneous voltage of the AC power source 1.
When an instantaneous voltage of the AC power source 1 is high, the increase rate of the current that flows in the inductor 41 is high. Therefore, the energy accumulated in the inductor 41 for a fixed period is large when the transistor 51 turns off, so that the charging voltage to the smoothing capacitor 55 increases. The input current becomes large in accordance with the instantaneous voltage of the AC power source 1.
Therefore, input current with a waveform similar to the voltage waveform of the AC power source 1 can be obtained. Thus, the input power factor as to the power source can be high and the harmonic components of the input current from the power source can be reduced.
However, the conventional ballast described above poses a problem in that the configuration is complex so that it can result in a large-scale and expensive device. Furthermore, when a source voltage is applied to the ballast via a dimmer, the voltage of the smoothing capacitor is likely to drop so that the operation becomes unstable. For this reason, a dimmer is difficult to use with the device.